San Francisco is among the cleanest counties, without a single day that exceeded federal ozone pollution standards in 2011.

Photo: Brant Ward, The Chronicle

San Francisco is among the cleanest counties, without a single day...

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California has had huge reductions in car exhaust and soot, particularly over the past decade, making for cleaner air.

Photo: Brant Ward, The Chronicle

California has had huge reductions in car exhaust and soot,...

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A view of San Francisco from the air with Coit tower in the foreground taken earlier this month. The San Francisco metropolitan area has dropped out of the 25 most polluted regions in the nation according to the American Lung Association State of the Air report Wednesday April 25, 2012.

Photo: Brant Ward, The Chronicle

A view of San Francisco from the air with Coit tower in the...

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A jogger runs past the Bay Bridge on The Embarcadero in San Francisco, Calif. on Wednesday, April 25, 2012. The San Francisco metropolitan area has dropped off the list of the top 25 most polluted regions in the nation, according to the American Lung Association's annual report on air quality and four of the nine cleanest counties in the state are in the Bay Area.

Photo: Paul Chinn, The Chronicle

A jogger runs past the Bay Bridge on The Embarcadero in San...

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Hikers walk past poppies in full bloom at the Berkeley Marina on Wednesday, April 25, 2012. The San Francisco metropolitan area has dropped off the list of the top 25 most polluted regions in the nation, according to the American Lung Association's annual report on air quality and four of the nine cleanest counties in the state are in the Bay Area.

Photo: Paul Chinn, The Chronicle

Hikers walk past poppies in full bloom at the Berkeley Marina on...

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Don Cowles prepares to launch his plane off an east bay hill as storm clouds appear on the horizon. The San Francisco metropolitan area has dropped out of the 25 most polluted regions in the nation according to the American Lung Association State of the Air report Wednesday April 25, 2012.

The San Francisco metropolitan area has dropped off the list of the top 25 most polluted regions in the nation, the American Lung Association said in a report Wednesday.

The State of the Air report ranked regions across the United States on particle and ozone pollution, finding that three of the nine least smoggy counties in California are located in the Bay Area and that, with a few exceptions, the air in Northern California is safe to inhale.

That, however, is not a license to breathe easy. California still ranks among the most polluted places in the nation, according to the report.

"Air pollution remains a serious health threat to too many Californians," said Jane Warner, president and chief executive officer of the American Lung Association in California. "We're making real and steady progress in the fight for clean air, but unhealthy levels of air pollution still exist, putting the health of California residents at risk."

The comprehensive list of the cleanest and most polluted areas in the country used data from the U.S. Environmental Protection Agency to calculate ozone levels, which primarily come from vehicle tailpipes, and particle pollution, most of which comes from burning wood and fossil fuels.

The report, which has been issued every year since 2000, has regularly shown California air to be among the most polluted in the United States, mainly because the warm, dry climate tends to trap smog, especially in Southern California and in the Central Valley.

Huge reductions

Lung association analysts gave credit Wednesday to California for huge reductions in car exhaust and soot, particularly over the past decade. In January, the California Air Resources Board approved regulations that will cut in half current greenhouse gas emissions and mandate production of more than a million zero-emission vehicles by 2025. The Bay Area Air Quality Management District has implemented strict wood-burning regulations during designated Spare the Air days.

The result, according to the lung association report, is that San Francisco is now among the cleanest counties in the country in terms of smog. The city and county did not record a single day that exceeded the federally designated ozone pollution standard last year. Marin, Sonoma, Santa Cruz, Mendocino, Lake, Humboldt, Glenn and Siskiyou counties were also on the ozone honor list.

The report said annual levels of particle pollution in Bay Area communities have dropped an average of between 15 and 30 percent since 2004. There has been a 51 percent decline in the number of days with unhealthy ozone levels since 2000.

"The Bay Area has made really dramatic progress in reducing both ozone and particle pollution," said Jenny Bard, the advocacy manager for the Lung Association in California.

There's more to do

There is nevertheless a lot more that can be done, Bard said.

The San Francisco metropolitan area, which includes San Jose and Oakland, only moved three spaces over the past year, from 24th to 27th worst, in the number of fine particles wafting in the air. It was the 33rd smoggiest region out of 232 metro areas in the country.

Much of the local smog can be blamed on car-happy Santa Clara County, which recorded seven days last year that exceeded federal ozone standards, the worst in the Bay Area. San Franciscans should not boast, though, because most of their pollution blows away and fouls other people's air, Bard said.

The state as a whole isn't looking particularly good either. The State of the Air report said more than 90 percent of Californians live in counties that received a failing grade for either ozone or particle pollution. No California municipality was among the top 25 cleanest cities, a list that ranked Santa Fe, N.M., No. 1.

California, in fact, had nine of the 10 most ozone-polluted metro areas. As usual, the Los Angeles metropolitan area, which includes Riverside, San Bernardino and Orange counties, had the worst ozone pollution in the country. San Bernardino County alone had 127 bad air days.

The other big smog areas were all in Southern California and the lower Central Valley, with the exception of San Luis Obispo, which is on the Central Coast.

"Ozone and particle pollution contribute to thousands of hospitalizations, emergency room visits, and deaths every year," said Kari Nadeau, an associate professor of immunology and allergy at Stanford Medical School. "Cleaner air can save lives and can lead to better lives for our children."

How's your air?

People interested in their local rankings for air pollution can go to stateoftheair.org and click on their ZIP code.